The properties of polymers (i.e., plastics) make them ideal substrate materials for use in many applications. In particular, optically transparent plastics such as polycarbonate, CR-39.RTM. (allyl diglycol carbonate) and acrylics have become widely accepted materials for use as optical lenses because of their light weight and ease of molding compared to glass. Polycarbonate also has superior impact resistance compared to glass. However, a major drawback with plastic substrates, particularly polycarbonate, is their poor scratch and abrasion resistance.
Various methods have been employed to enhance the abrasion wear resistance of plastic substrates. For example, many commercial plastic opthalmic and sunglass lenses are coated with either organic acrylate-type polymer coatings or polysiloxane-type polymer coatings. (See, S. Herbert, Industrial Diamond Review, February 1984, at p.77.) Although these polymer coatings offer a significant improvement in abrasion resistance relative to the uncoated plastic lens, the perceived abrasion resistance of the coated plastic lens compared to a glass lens is still poor.
Glass and silicon dioxide have also been employed as coatings on plastic substrates to improve the abrasion resistance. Illustrative are Great Britain Patent No. 1,335,065, and U.S. Pat. Nos. 3,458,342 and 4,328,646. However, the absence of an intermediate layer between the glass or silicon dioxide layer and the plastic substrate often results in the glass or silicon dioxide coating spalling or cracking when the substrate is subjected to thermal cycling.
Several prior art techniques disclose the use of an intermediate layer between the substrate and the glass or silicon dioxide outer layer to improve adhesion of the outer layer. For example, U.S. Pat. No. 4,190,681, issued to Hall, et al., discloses an evaporative deposition technique of a glass layer disposed on top of an intermediate layer of an acrylic-type polymer which has in turn been coated onto a polycarbonate substrate. U.S. Pat. No. 4,200,681, also issued to Hall, et al., discloses the vapor deposition of a top layer of silicon dioxide onto an intermediate primer layer which in turn has been deposited on the surface of a polycarbonate substrate. However, this particular evaporative technique of applying a layer of silicon dioxide is often undesirable for several reasons. First, this technique suffers from inadequate adhesion of the silicon dioxide or glass layer, due to (i) the relatively low reactivity of the evaporated silicon oxide film-forming species, and (ii) insufficient bond strength between the silicon dioxide layer and the underlying carbonaceous acrylic polymer layer. Second, the individual particles of silicon dioxide may evaporate and later condense on the coating surface at rates which vary with the particular site of deposition, resulting in a non-uniform glass surface often characterized by pits, pinholes, and other imperfections.
U.S. Pat. No. 3,713,869, teaches the deposition of an intermediate layer polymerized by glow discharge onto a polycarbonate surface. A hard inorganic glass layer is then vaporized by an electron beam gun onto the intermediate layer in a manner similar to that used by Hall, et al. European Patent Application No. 0,266,225 further discloses a coating in which the plastic substrate is first coated with a silicon-based layer which is overcoated by a top layer of silicon dioxide. U.S. Pat. No. 4,842,941 discloses a polycarbonate substrate with an interfacial layer of resinous composition, and an abrasion-resistant top layer applied by plasma-enhanced chemical vapor deposition. U.S. Pat. No. 4,341,841 discloses an article with a multilayer protective coating, comprising a substrate and two protective layers, one being a vacuum coated ceramic layer, and one being a resinous layer, coated in any order. Although each of these prior art techniques have resulted in limited improvement in the adhesion between the glass or silicon dioxide coating and the substrate, and ultimately the wear resistance of the coated substrate product, the prior art techniques do not contemplate a diamond-like carbon ("DLC") outer layer, nor address the optimum method for obtaining an adherent DLC layer on a polycarbonate substrate.
In all of the aforementioned prior art techniques, the abrasion resistance of the coated plastic substrate has been unsatisfactory because of the limited hardness of the silicon dioxide or glass coating. Additional problems are also encountered by thin oxide coatings. Due to incomplete oxidation or inhomogeneous chemical bonding which is characteristic of oxide films, the films are susceptible to chemical reaction and damage by salt water. This is a particular disadvantage for eyeglass or sunglass lenses which are exposed to perspiration or ocean spray. Finally, under thermal cycling or flexing, the oxide coatings are susceptible to cracking and peeling.
There are also teachings of "mixed phase" or "gradient" inorganic hard coatings for plastic substrates which can be found in the prior art. For example, U.S. Pat. No. 4,830,873, issued to Benz, et al., discloses a process for applying a transparent layer onto the surface of a plastic element by first polymerizing an organic vapor and subsequently introducing additives such as oxygen, hydrocarbon compounds, or nitrogen-containing compounds to the vapor to form a layer with increased hardness. International Patent Application No. WO89/01957 discloses a method for depositing an abrasion-resistant coating comprising the plasma-enhanced chemical vapor deposition of a coating characterized by a gradual transition from a composition consisting essentially of an interfacial material to a composition consisting essentially of an abrasion-resistant material. Further, U.S. Pat. No. 4,777,090, discloses a product which has a disordered carbon coating at the substrate-coating interface and a relatively ordered portion composed of either carbon or silicon dioxide away from the substrate-coating interface. These prior art techniques similarly do not teach the deposition of a hard DLC layer, nor discuss the formation of a discrete multilayer coating structure with a polycarbonate substrate and an adherent DLC outer layer.
There are, however, several prior art references which teach the direct deposition of hard DLC films onto plastic substrates to improve abrasion resistance. Illustrative are U.S. Pat. Nos. 4,663,183, 4,770,940, 4,783,361, 4,698,256 and 4,877,677, and European Application Nos. 0,280,215 and 0,278,480. These prior art references do not teach the use of interlayer materials which have been found to be essential to achieve satisfactory adherence of the DLC film.
There are also prior art references which teach the deposition of hard DLC films onto plastic substrates with the use of limited interlayer materials. For example, U.S. Pat. No. 4,661,409, issued to Kieser, et al., discloses a substrate having an amorphous carbon coating and an adhesion-mediating interlayer of a siloxane or silazane polymer between the carbon film and the substrate. U.S. Pat. No. 4,569,738, also issued to Kieser, et al., discloses a microwave discharge process for depositing the siloxane or silazane polymer and amorphous carbon layers on the substrate. However, in each of these references only intermediate layers of microwave discharge-deposited siloxane and silazane polymers are discussed.
Although DLC coatings possess excellent optical properties and exhibit excellent resistance to abrasion and chemical attack, DLC coatings have not been widely applied to plastic substrates (including lenses) to date for several reasons. First, while DLC coatings are indeed very hard, they are also brittle when thin. Thus, when applied to soft substrates such as plastics, the DLC coating can crack and/or be crushed into the substrate when a high load or force is applied to the surface of the substrate. This mechanism is also responsible for the apparent scratching of the DLC coating from the surface of plastic substrates in severe abrasive environments.
Second, the adhesion of DLC coatings to plastic substrates has been poor due to the high internal stress associated with the DLC coatings. This poor adhesion has been especially evident during cooling of DLC coated plastic substrates from elevated temperatures. There is also a significant difference in the thermal expansion coefficients between the DLC coating and the plastic substrate. Thus, during thermal cycling the weak adhesive bonding strength at the DLC-plastic interface is overwhelmed by the forces generated by expansion and contraction, and hence, the DLC coating cracks and delaminates.
It is therefore an object of the present invention to provide a plastic substrate with hard coated surface layers, such as diamond-like carbon, firmly adhered thereto, thereby to prevent undesirable separation or crack formation, while at the same time providing excellent hardness and resistance to abrasion, chemical attack and impact.
It is a further object of the present invention to provide an abrasion-resistant plastic substrate with increased ease of cleaning.
It is a further object of the present invention to provide a plastic substrate with a diamond-like hard carbon coating which combines the ability to reflect decorative colors without sacrificing the aforementioned objects.
It is a further object of the present invention to provide a method for depositing an adherent coating incorporating high refractive index diamond-like hard carbon layers in an alternating layer stack along with at least one other material of substantially different refractive index in order to produce thin film interference coatings such as quarter wavelength stacks.
It is a further object of this invention to provide a low cost and efficient process for producing a plastic substrate with superior abrasion wear resistance and reduced chemical reactivity.